Wood continues to be the most commonly used framing material for the construction of residential properties. Its weakness to termite attack in some countries has been lessened by treatment of wood with insecticides. Application methods and approved chemicals vary dramatically through out the world.
Softwood timbers, pinus radiata, pinus elliotti, and pinus carribea used as framing timber in Australia are susceptible to termite attack. Changes to government regulations have limited the use of soil poisoning agents (banning of organo-chloride insecticides), which has led to a higher incidence of termite attack of timber framed houses.
Many countries including Australia and the USA are struggling to find suitable cost-effective methods to combat this ever-increasing risk of termite attack.
One of the strategies to combat termite attack of softwood frames is the treatment of the timber with insecticides or more broad-spectrum wood preservatives.
In Australia, for example, treatment of timber is covered by the Australian standard AS 1604-2000/. Hazard class H2 is defined for the biological hazard—borer and termites. Retention is measured in mass/mass (% m/m).
The approved chemicals are shown in the following table.
TABLE 1Minimum preservative retention in the penetration zone-HazardClass 2 (H2)WaterborneCopperAmmoniacalchromecopperarsenicquaternary(CU +(Cu +Light organic solvent preservativesCr + As)DDAC)PermethrinCypermethrinDeltamethrin0.320% m/m0.35% m/m0.020% m/m0.030% m/m.0020% m/mPenetration is defined under the standard as—“All preservative-treated wood shall show evidence of distribution of the preservative in the penetration zone in accordance with the following requirements:                (a) If the species of timber used is of natural durability class 1 or 2, the preservative shall penetrate all the sapwood. Preservative penetration of the heartwood is not required.        (b) If the species of timber used is of natural durability class 3 or 4, the preservative shall penetrate all of the sapwood and, in addition one of the following requirements shall apply.                    (i) Where the lesser cross-sectional dimension is greater than 35 mm, the penetration shall be not less than 8 mm from any surface. Where the lesser cross-sectional dimension is equal or less then 35 mm, the penetration shall be not less than 5 mm from any surface.            (ii) Unpenetrated heartwood shall be permitted, provided that it comprises less than 20% of the cross-section of the piece and does not extend more than halfway through the piece from one surface to the opposite surface and does not exceed half the dimension of the side in the cross-section on which it occurs.”                        
In order to provide for penetration of the preservative, a carrier must be used. As shown in the Australian standard, the carriers currently available are waterborne or solvent borne systems.
Waterborne carriers swell wood and hence timber thus treated needs to be re-dried prior to use in service. Australian Standards specify the maximum moisture content of pine framing. This level is around 12-14% moisture content.
The process sequence is:                Dry wood→treat→re-dry wood        
Solvent borne preservatives because they are non-polar do not raise the moisture content and hence do not swell the wood.
The process sequence is:                Dry wood→solvent treat        
The disadvantage of this treatment is the high cost of solvents and potential environmental concerns with volatile organic compounds (VOC's) being released into the atmosphere.
Application of the insecticides to wood is normally carried out by a batch process involving a pressure vessel. For water-borne preservatives a vacuum pressure process (Bethell or full cell) is used. This ensures, providing the wood is dry, complete sapwood penetration and adequate heartwood penetration if required.
For LOSP (light organic solvent preservatives) a double vacuum process ensures penetration to AS 1604-2000.
Pressure plants are expensive to construct, and being batch processes, conventional treatments do not match well with continuous sawmill production and require a high level of operator control to maintain costs.
The present invention seeks to overcome at least some of the disadvantages of the prior art or at least provide a commercial alternative thereto.